This invention relates to an interference system and a semiconductor exposure apparatus having the same. Particularly, the present invention is suitably applicable to a system, such as a wavefront aberration measuring machine for a projection lens used in a semiconductor device manufacturing exposure apparatus, for example, in which the length of the optical path is large and, additionally, high precision wavefront measurement is required through the wavelength of light rays usable for the measurement is restricted, and also in which the wavefront aberration of the projection lens should be measured while the lens is kept mounted on the apparatus.
Conventionally, a transmission wavefront of a projection lens in a semiconductor device manufacturing exposure apparatus is measured, in many cases, by using a Fizeau type interferometer in which most of the light path for reference light and detection light is consistent, for attaining high precision measurement. In the wavefront measurement by using such a Fizeau type interferometer, a lens (projection lens), which is the subject to be measured, is placed between a Fizeau plane (or surface) and a reflection reference mirror surface. The transmission wavefront of the subject to be measured is measured on the basis of interference of the two lights reflected by these two surfaces. For this reason, the light source to be used in a Fizeau type interferometer must be one which can emit light having a coherency more than twice that of the optical path length between the Fizeau plane and the reflection reference mirror surface. In addition to this, the wavelength of light used for the wavefront measurement must be the same as or very close to the wavelength of exposure light to be used in the semiconductor exposure apparatus. For example, for measurement of the wavefront aberration of a projection lens where g-line light (435 nm) is used as exposure light, a HeCd laser which emits light having a wavelength of 442 nm may be used. For measurement of the wavefront aberration of a projection lens where i-line light (365 nm) is used as exposure light, an Ar ion laser which emits light having a wavelength of (365 nm) may be used. For measurement of the wavefront aberration of a projection lens when a KrF excimer laser (248 nm) is used as exposure light, a second harmonic of an Ar ion laser which emits light having a wavelength of 248 nm may be used. However, for measurement of the wavefront aberration of a projection lens when an ArF excimer laser (193 nm) is used as exposure light, a light source having a similar wavelength and a large coherence length is not currently available. Therefore, it is not possible to make a Fizeau type interferometer and, as a consequence, a Twyman-Green type interferometer is used. The latter is arranged so that, for the measurement of wavefront aberration, the optical path lengths for the reference light and the detection light are made equal to each other, such that the measurement is attainable even with the use of a light source having a short coherence length.
A reduction in size of a semiconductor device pattern requires a higher optical performance of a projection lens. Also, it needs high precision measurement for an interferometer for the lens measurement, and the projection lens itself should keep a very accurate optical performance. This means that the transmission wavefront of a projection lens should desirably be measured while the lens is kept mounted on a semiconductor exposure apparatus. However, since in a Twyman-Green type interferometer the reference light and the detection light pass along different optical paths, there is a disadvantage that it is easily influenced by an external disturbance. Additionally, because of the necessity of the reference light, the size of the interferometer becomes large, which is very inconvenient when the interferometer is mounted on the semiconductor exposure apparatus.
It is accordingly an object of the present invention to provide a Fizeau type interferometer system capable of measuring wavefront aberration of a projection lens very accurately even when a light source which emits light of a short coherence length is used, and also to provide an exposure apparatus having the same.
It is another object of the present invention to provide an exposure apparatus with a Fizeau type interferometer, by which the transmission wavefront of a projection lens can be measured in a state that the projection lens is kept mounted.